6.4.3 Universal character names
UCN allows the use of any character in a C source, not just English characters. A UCN has the format:
\u 4_hex_digits_value
\U 8_hex_digits_value
A UCN must not specify a value less than 00A0 other than 0024 ($), 0040 (@), or 0060 (?), nor a value in the range D800 through DFFF inclusive.
UCN may be used in identifiers, character constants, and string literals to designate characters that are not in the C basic character set.
The UCN \unnnnnnnn designates the character whose eight-digit short identifier (as specified by ISO/IEC 10646) is nnnnnnnn. Similarly, the universal character name ¯nnnn designates the character whose four-digit short identifier is nnnn (and whose eight-digit short identifier is 0000nnnn).